Communication satellites receive and transmit radio signals from and to the surface of the Earth. Although Earth-orbiting communications satellites have been in use for many years, providing adequate cooling and heat distribution for the thermally sensitive electronics components onboard such satellites continues to be a problem.
There are two primary sources of heat with which a satellite's thermal systems must contend. One source is solar radiation. Solar radiation can be absorbed by thermal insulation shields or readily reflected away from the satellite by providing the satellite with a suitably reflective exterior surface. A second source of heat is the electronics onboard the satellite. The removal of electronics-generated heat is more problematic since such heat must be collected from various locations within the satellite, transported to a site at which it can be rejected from the satellite, and then radiated into space.
Passive thermal panels can be used to dissipate heat from satellites. In one configuration, the passive thermal panel includes a honeycomb core having heat pipes embedded therein. A heat pipe is a closed chamber, typically in the form of tube, having an internal capillary structure which is filled with a working fluid. The operating-temperature range of the satellite sets the choice of working fluid; ammonia, ethane and propylene are typical choices. Heat input (i.e., from heat-generating electronics) causes the working fluid to evaporate. The evaporated fluid carries the heat towards a colder heat-output section, where heat is rejected as the fluid condenses. The rejected heat is absorbed by the cooler surfaces of the heat-output section and then radiated into space. The condensate returns to the heat input section (near to heat-generating components) by capillary forces to complete the cycle.
The honeycomb core is typically a low strength, lightweight material. For this reason among any others, thin, stiff panels or “skins” are disposed on both major surfaces of the honeycomb core. The core is thus “sandwiched” between the skins. The strength of this composite is dependent largely on: (1) the outer skins and (2) an adhesive layer that bonds the honeycomb core and the skins. The panels are very expensive and labor intensive to manufacture but are required nearly everywhere that there are out-of-plane loads or modal concerns.
A second configuration of a passive thermal panel is simply a solid metallic skin. Such skins are, however, structurally inefficient for use in satellites since the skins' bending stiffness scales with the cube of its thickness. Unless expensive and heavy stiffeners are added to increase bending stiffness, such solid skins can only be used over short spans or with very little mass (i.e., structures) mounted thereto.
A need therefore remains for improvements in passive thermal panels for use in satellites.